Jami` at-Tirmidhi 668
جامع الترمذي#668SahihIbn Umar narrated that :Umar gave a horse to be used in the cause of Allah. Then he saw it being sold, so he wanted to buy it, but the Prophet said: "Do not take back what you have given in charity
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 7:52